Science
by Brian Greene
Can one theory explain it all?
Physics rests on two theories. General relativity covers gravity and the very large. Quantum mechanics covers the very small. Each works superbly, but where both are needed, as at the centre of a black hole, their maths breaks down. Brian Greene explains how string theory tries to join them.
Hear a sample · read by Laura
Quantum mechanics says nothing at small scales ever sits perfectly still. Greene explains that at the tiniest distances, space itself should churn wildly, a froth physicists call quantum foam. General relativity needs smooth, gently curving space, so when you combine the two there, the calculations give infinite, meaningless answers.
String theory replaces point-like particles with tiny vibrating strings. Greene compares them to violin strings, where each pattern of vibration gives a different note. Here each pattern shows up as a different particle, with its own mass and charge. One pattern matches the particle expected to carry gravity.
The maths of string theory only works if space has more dimensions than the three we know. In the version Greene describes, there are nine of space and one of time. The extra six would be curled up far too small to notice, into shapes that decide which particles exist.
Greene is open that no experiment has confirmed string theory. Strings, if they exist, are far too small for any machine to see directly. He hoped for indirect signs, like the partner particles supersymmetry predicts. None had turned up when he wrote, and none has since.
The conflict
Relativity and quantum mechanics both work, but they clash when you try to join them.
Strings
String theory says the smallest things are tiny vibrating strings of energy.
Notes
Each kind of particle is a string playing a different note.
Extra dimensions
The theory needs extra dimensions, curled up too small for us to see.
One theory
If it holds up, gravity and quantum physics finally fit together in one theory.
A story from the book
For years there were five different versions of string theory, awkward for something meant to be the one theory of everything. In 1995 Edward Witten argued they were all parts of a single bigger theory, now called M-theory, which needs one more dimension of space.
Hot take · true or myth?
Relativity and quantum mechanics fit together neatly.
Myth. Each is superbly accurate on its own. Combine them to describe something tiny and very heavy, like the centre of a black hole, and the maths gives infinite, meaningless answers.
One thing to try
Look at a garden hose from the far end of the garden. It looks like a line, with one dimension. Walk up close and you can see it also has a round surface an ant could walk around. That's Greene's picture of how extra dimensions could hide.
Knotd has the key ideas from 250+ non-fiction books, each in a few minutes, in plain words. The Elegant Universe comes with a Hot Take round and a Name That Idea round to make it stick.
Get the whole book on KnotdOpen in the appCan one theory explain it all? Physics rests on two theories. General relativity covers gravity and the very large. Quantum mechanics covers the very small. Each works superbly, but where both are needed, as at the centre of a black hole, their maths breaks down. Brian Greene explains how string theory tries to join them.
Trouble at tiny scales; Tiny vibrating strings; More dimensions than we see; Still waiting for evidence.
Look at a garden hose from the far end of the garden. It looks like a line, with one dimension. Walk up close and you can see it also has a round surface an ant could walk around. That's Greene's picture of how extra dimensions could hide.
Yes. Knotd has the key ideas from The Elegant Universe in a few minutes, in plain words, plus a Hot Take round and a Name That Idea round to test them. It's one of 250+ non-fiction books in the app.
